Changes in excitability of human cutaneous afferents following prolonged high-frequency stimulation

Prolonged high-frequency stimulation of cutaneous nerves can result in paraesthesiae that begin 20 to 30 s after the end of the train and last for 5 to 10 min. In the present experiments the effects of such stimulation on the excitability of human cutaneous afferents and on their refractory and supernormal periods were measured to determine whether these changes could explain the postactivation paraesthesiae. Attention was focused on the axons of lowest threshold (1.0-1.5 T) in the compound sensory action potential evoked by stimulating the digital nerves of the index or middle fingers. Repetitive activation produced two opposing effects on the excitability of low-threshold cutaneous afferents. Following stimulus trains of short duration (1-5 min) the dominant effect was a long-lasting decrease in excitability, such that the amplitude of a test afferent volley was always less than before stimulation. With these trains, no subject experienced paraesthesiae. For 10 min after stimulus trains lasting longer than 7 to 12 min the dominant effect was an increase in excitability such that the amplitude of the test volley was greater than before stimulation. Within this interval, following such trains, subjects experienced paraesthesiae. The extent and duration of supernormality induced by a supramaximal conditioning stimulus were greatly increased by stimulation for 1 min. Following stimulation for 10 min, the degree of supernormality of the enhanced test volley was much the same as before stimulation, but was inappropriately high for the size of the test volley. The sum total of the excitability change and the change in supernormality resulted in a larger potential after stimulation, whether the train lasted 1 min or 10 min. It is concluded that the postactivation changes in axonal excitability could predispose the most excitable axons to generate ectopic impulses and, thereby, to produce paraesthesiae.